20 : 6/ Photosynthesis 



375 



If the flashes are shortened to 0.5 msec, it is found that one flash does 

 not result in the liberation of oxygen from whole cells. A flash of the 



480 560 640 



Wavelength (m\i) 



720 



Figure 10. Action spectrum of chlorella. Note the extremely 

 flat curve as compared with those of Figures 7 and 9. After 

 R. Emerson and C. Lewis, "Dependence of the Quantum 

 Yield of Photosynthesis on Wave Length of Light," American 

 J. Botany 30: 165 (1943). 



same total number of photons but spread over a 25 msec interval does 

 result in the liberation of oxygen. (In the Hill reaction, even the short 

 flash results in the liberation of oxygen.) Two consecutive flashes lead 

 to a greater evolution of oxygen than twice that produced by one flash. 

 Thus, each flash leads to the production of stable intermediates. It is 

 not clear in the flashing-light experiments whether this is merely con- 

 verting all the ADP to ATP the first flash, or whether it is the same 

 intermediate indicated by the light-saturation experiments. 



The most direct evidence for the existence of an intermediate com- 

 pound in an excited state comes from studies of electron spin resonance. 

 This technique is discussed in Chapter 31, "Magnetic Measurements." 

 Essentially, it is based on the inherent magnetic moment (spin) possessed 

 by all electrons. In most organic compounds encountered in biological 

 materials, the electrons exist as pairs with their magnetic moments 

 opposing one another and thereby cancelling. There also exist certain 

 excited forms of compounds normally expected to have paired electrons 

 only, in which one electron is missing, or an extra electron is present. 

 In either case, the compound has a net magnetic moment and is called a 

 free radical. Magnetic studies with chloroplast materials indicate that 

 such free radicals form when light falls on the chloroplast. The very 

 nature of the magnetic studies makes it impossible to tell just what 

 compound contains the unpaired electron. In contrast to most free 



